C++ LVILatticeVal类代码示例

// If we can determine a constant range for the value Val in the context
// provided by the instruction BBI, then merge it into BBLV. If we did find a
// constant range, return true.
void LazyValueInfoCache::mergeAssumeBlockValueConstantRange(Value *Val,
                                                            LVILatticeVal &BBLV,
                                                            Instruction *BBI) {
  BBI = BBI ? BBI : dyn_cast<Instruction>(Val);
  if (!BBI)
    return;

  for (auto &AssumeVH : AC->assumptions()) {
    if (!AssumeVH)
      continue;
    auto *I = cast<CallInst>(AssumeVH);
    if (!isValidAssumeForContext(I, BBI, DL, DT))
      continue;

    Value *C = I->getArgOperand(0);
    if (ICmpInst *ICI = dyn_cast<ICmpInst>(C)) {
      LVILatticeVal Result;
      if (getValueFromFromCondition(Val, ICI, Result)) {
        if (BBLV.isOverdefined())
          BBLV = Result;
        else
          BBLV.mergeIn(Result);
      }
    }
  }
}

bool LazyValueInfoCache::solveBlockValuePHINode(LVILatticeVal &BBLV,
                                                PHINode *PN, BasicBlock *BB) {
  LVILatticeVal Result;  // Start Undefined.

  // Loop over all of our predecessors, merging what we know from them into
  // result.
  bool EdgesMissing = false;
  for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
    BasicBlock *PhiBB = PN->getIncomingBlock(i);
    Value *PhiVal = PN->getIncomingValue(i);
    LVILatticeVal EdgeResult;
    EdgesMissing |= !getEdgeValue(PhiVal, PhiBB, BB, EdgeResult);
    if (EdgesMissing)
      continue;

    Result.mergeIn(EdgeResult);

    // If we hit overdefined, exit early.  The BlockVals entry is already set
    // to overdefined.
    if (Result.isOverdefined()) {
      DEBUG(dbgs() << " compute BB '" << BB->getName()
            << "' - overdefined because of pred.\n");
      
      BBLV = Result;
      return true;
    }
  }
  if (EdgesMissing)
    return false;

  // Return the merged value, which is more precise than 'overdefined'.
  assert(!Result.isOverdefined() && "Possible PHI in entry block?");
  BBLV = Result;
  return true;
}

Constant *LazyValueInfo::getConstant(Value *V, BasicBlock *BB) {
  LVILatticeVal Result = getCache(PImpl).getValueInBlock(V, BB);
  
  if (Result.isConstant())
    return Result.getConstant();
  if (Result.isConstantRange()) {
    ConstantRange CR = Result.getConstantRange();
    if (const APInt *SingleVal = CR.getSingleElement())
      return ConstantInt::get(V->getContext(), *SingleVal);
  }
  return 0;
}

/// \brief Compute the value of Val on the edge BBFrom -> BBTo or the value at
/// the basic block if the edge does not constrain Val.
bool LazyValueInfoCache::getEdgeValue(Value *Val, BasicBlock *BBFrom,
                                      BasicBlock *BBTo, LVILatticeVal &Result,
                                      Instruction *CxtI) {
  // If already a constant, there is nothing to compute.
  if (Constant *VC = dyn_cast<Constant>(Val)) {
    Result = LVILatticeVal::get(VC);
    return true;
  }

  if (getEdgeValueLocal(Val, BBFrom, BBTo, Result)) {
    if (!Result.isConstantRange() ||
        Result.getConstantRange().getSingleElement())
      return true;

    // FIXME: this check should be moved to the beginning of the function when
    // LVI better supports recursive values. Even for the single value case, we
    // can intersect to detect dead code (an empty range).
    if (!hasBlockValue(Val, BBFrom)) {
      if (pushBlockValue(std::make_pair(BBFrom, Val)))
        return false;
      Result.markOverdefined();
      return true;
    }

    // Try to intersect ranges of the BB and the constraint on the edge.
    LVILatticeVal InBlock = getBlockValue(Val, BBFrom);
    mergeAssumeBlockValueConstantRange(Val, InBlock, BBFrom->getTerminator());
    // See note on the use of the CxtI with mergeAssumeBlockValueConstantRange,
    // and caching, below.
    mergeAssumeBlockValueConstantRange(Val, InBlock, CxtI);
    if (!InBlock.isConstantRange())
      return true;

    ConstantRange Range =
      Result.getConstantRange().intersectWith(InBlock.getConstantRange());
    Result = LVILatticeVal::getRange(Range);
    return true;
  }

  if (!hasBlockValue(Val, BBFrom)) {
    if (pushBlockValue(std::make_pair(BBFrom, Val)))
      return false;
    Result.markOverdefined();
    return true;
  }

  // If we couldn't compute the value on the edge, use the value from the BB.
  Result = getBlockValue(Val, BBFrom);
  mergeAssumeBlockValueConstantRange(Val, Result, BBFrom->getTerminator());
  // We can use the context instruction (generically the ultimate instruction
  // the calling pass is trying to simplify) here, even though the result of
  // this function is generally cached when called from the solve* functions
  // (and that cached result might be used with queries using a different
  // context instruction), because when this function is called from the solve*
  // functions, the context instruction is not provided. When called from
  // LazyValueInfoCache::getValueOnEdge, the context instruction is provided,
  // but then the result is not cached.
  mergeAssumeBlockValueConstantRange(Val, Result, CxtI);
  return true;
}

Constant *LazyValueInfo::getConstant(Value *V, BasicBlock *BB,
                                     Instruction *CxtI) {
  LVILatticeVal Result =
      getCache(PImpl, AC, DL, DT).getValueInBlock(V, BB, CxtI);

  if (Result.isConstant())
    return Result.getConstant();
  if (Result.isConstantRange()) {
    ConstantRange CR = Result.getConstantRange();
    if (const APInt *SingleVal = CR.getSingleElement())
      return ConstantInt::get(V->getContext(), *SingleVal);
  }
  return nullptr;
}

/// Determine whether the specified value is known to be a
/// constant on the specified edge. Return null if not.
Constant *LazyValueInfo::getConstantOnEdge(Value *V, BasicBlock *FromBB,
                                           BasicBlock *ToBB,
                                           Instruction *CxtI) {
  const DataLayout &DL = FromBB->getModule()->getDataLayout();
  LVILatticeVal Result =
      getCache(PImpl, AC, &DL, DT).getValueOnEdge(V, FromBB, ToBB, CxtI);

  if (Result.isConstant())
    return Result.getConstant();
  if (Result.isConstantRange()) {
    ConstantRange CR = Result.getConstantRange();
    if (const APInt *SingleVal = CR.getSingleElement())
      return ConstantInt::get(V->getContext(), *SingleVal);
  }
  return nullptr;
}

/// \brief Compute the value of Val on the edge BBFrom -> BBTo, or the value at
/// the basic block if the edge does not constraint Val.
bool LazyValueInfoCache::getEdgeValue(Value *Val, BasicBlock *BBFrom,
                                      BasicBlock *BBTo, LVILatticeVal &Result) {
  // If already a constant, there is nothing to compute.
  if (Constant *VC = dyn_cast<Constant>(Val)) {
    Result = LVILatticeVal::get(VC);
    return true;
  }

  if (getEdgeValueLocal(Val, BBFrom, BBTo, Result)) {
    if (!Result.isConstantRange() ||
      Result.getConstantRange().getSingleElement())
      return true;

    // FIXME: this check should be moved to the beginning of the function when
    // LVI better supports recursive values. Even for the single value case, we
    // can intersect to detect dead code (an empty range).
    if (!hasBlockValue(Val, BBFrom)) {
      BlockValueStack.push(std::make_pair(BBFrom, Val));
      return false;
    }

    // Try to intersect ranges of the BB and the constraint on the edge.
    LVILatticeVal InBlock = getBlockValue(Val, BBFrom);
    if (!InBlock.isConstantRange())
      return true;

    ConstantRange Range =
      Result.getConstantRange().intersectWith(InBlock.getConstantRange());
    Result = LVILatticeVal::getRange(Range);
    return true;
  }

  if (!hasBlockValue(Val, BBFrom)) {
    BlockValueStack.push(std::make_pair(BBFrom, Val));
    return false;
  }

  // if we couldn't compute the value on the edge, use the value from the BB
  Result = getBlockValue(Val, BBFrom);
  return true;
}

bool LazyValueInfoCache::solveBlockValueNonLocal(LVILatticeVal &BBLV,
                                                 Value *Val, BasicBlock *BB) {
  LVILatticeVal Result;  // Start Undefined.

  // If this is a pointer, and there's a load from that pointer in this BB,
  // then we know that the pointer can't be NULL.
  bool NotNull = false;
  if (Val->getType()->isPointerTy()) {
    if (isKnownNonNull(Val)) {
      NotNull = true;
    } else {
      Value *UnderlyingVal = GetUnderlyingObject(Val);
      // If 'GetUnderlyingObject' didn't converge, skip it. It won't converge
      // inside InstructionDereferencesPointer either.
      if (UnderlyingVal == GetUnderlyingObject(UnderlyingVal, NULL, 1)) {
        for (BasicBlock::iterator BI = BB->begin(), BE = BB->end();
             BI != BE; ++BI) {
          if (InstructionDereferencesPointer(BI, UnderlyingVal)) {
            NotNull = true;
            break;
          }
        }
      }
    }
  }

  // If this is the entry block, we must be asking about an argument.  The
  // value is overdefined.
  if (BB == &BB->getParent()->getEntryBlock()) {
    assert(isa<Argument>(Val) && "Unknown live-in to the entry block");
    if (NotNull) {
      PointerType *PTy = cast<PointerType>(Val->getType());
      Result = LVILatticeVal::getNot(ConstantPointerNull::get(PTy));
    } else {
      Result.markOverdefined();
    }
    BBLV = Result;
    return true;
  }

  // Loop over all of our predecessors, merging what we know from them into
  // result.
  bool EdgesMissing = false;
  for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
    LVILatticeVal EdgeResult;
    EdgesMissing |= !getEdgeValue(Val, *PI, BB, EdgeResult);
    if (EdgesMissing)
      continue;

    Result.mergeIn(EdgeResult);

    // If we hit overdefined, exit early.  The BlockVals entry is already set
    // to overdefined.
    if (Result.isOverdefined()) {
      DEBUG(dbgs() << " compute BB '" << BB->getName()
            << "' - overdefined because of pred.\n");
      // If we previously determined that this is a pointer that can't be null
      // then return that rather than giving up entirely.
      if (NotNull) {
        PointerType *PTy = cast<PointerType>(Val->getType());
        Result = LVILatticeVal::getNot(ConstantPointerNull::get(PTy));
      }
      
      BBLV = Result;
      return true;
    }
  }
  if (EdgesMissing)
    return false;

  // Return the merged value, which is more precise than 'overdefined'.
  assert(!Result.isOverdefined());
  BBLV = Result;
  return true;
}

bool LazyValueInfoCache::solveBlockValue(Value *Val, BasicBlock *BB) {
  if (isa<Constant>(Val))
    return true;

  if (lookup(Val).count(BB)) {
    // If we have a cached value, use that.
    DEBUG(dbgs() << "  reuse BB '" << BB->getName()
                 << "' val=" << lookup(Val)[BB] << '\n');

    // Since we're reusing a cached value, we don't need to update the
    // OverDefinedCache. The cache will have been properly updated whenever the
    // cached value was inserted.
    return true;
  }

  // Hold off inserting this value into the Cache in case we have to return
  // false and come back later.
  LVILatticeVal Res;

  Instruction *BBI = dyn_cast<Instruction>(Val);
  if (!BBI || BBI->getParent() != BB) {
    if (!solveBlockValueNonLocal(Res, Val, BB))
      return false;
   insertResult(Val, BB, Res);
   return true;
  }

  if (PHINode *PN = dyn_cast<PHINode>(BBI)) {
    if (!solveBlockValuePHINode(Res, PN, BB))
      return false;
    insertResult(Val, BB, Res);
    return true;
  }

  if (AllocaInst *AI = dyn_cast<AllocaInst>(BBI)) {
    Res = LVILatticeVal::getNot(ConstantPointerNull::get(AI->getType()));
    insertResult(Val, BB, Res);
    return true;
  }

  // We can only analyze the definitions of certain classes of instructions
  // (integral binops and casts at the moment), so bail if this isn't one.
  LVILatticeVal Result;
  if ((!isa<BinaryOperator>(BBI) && !isa<CastInst>(BBI)) ||
     !BBI->getType()->isIntegerTy()) {
    DEBUG(dbgs() << " compute BB '" << BB->getName()
                 << "' - overdefined because inst def found.\n");
    Res.markOverdefined();
    insertResult(Val, BB, Res);
    return true;
  }

  // FIXME: We're currently limited to binops with a constant RHS.  This should
  // be improved.
  BinaryOperator *BO = dyn_cast<BinaryOperator>(BBI);
  if (BO && !isa<ConstantInt>(BO->getOperand(1))) {
    DEBUG(dbgs() << " compute BB '" << BB->getName()
                 << "' - overdefined because inst def found.\n");

    Res.markOverdefined();
    insertResult(Val, BB, Res);
    return true;
  }

  if (!solveBlockValueConstantRange(Res, BBI, BB))
    return false;
  insertResult(Val, BB, Res);
  return true;
}

static LazyValueInfo::Tristate
getPredicateResult(unsigned Pred, Constant *C, LVILatticeVal &Result,
                   const DataLayout *DL, TargetLibraryInfo *TLI) {

  // If we know the value is a constant, evaluate the conditional.
  Constant *Res = nullptr;
  if (Result.isConstant()) {
    Res = ConstantFoldCompareInstOperands(Pred, Result.getConstant(), C, DL,
                                          TLI);
    if (ConstantInt *ResCI = dyn_cast<ConstantInt>(Res))
      return ResCI->isZero() ? LazyValueInfo::False : LazyValueInfo::True;
    return LazyValueInfo::Unknown;
  }

  if (Result.isConstantRange()) {
    ConstantInt *CI = dyn_cast<ConstantInt>(C);
    if (!CI) return LazyValueInfo::Unknown;

    ConstantRange CR = Result.getConstantRange();
    if (Pred == ICmpInst::ICMP_EQ) {
      if (!CR.contains(CI->getValue()))
        return LazyValueInfo::False;

      if (CR.isSingleElement() && CR.contains(CI->getValue()))
        return LazyValueInfo::True;
    } else if (Pred == ICmpInst::ICMP_NE) {
      if (!CR.contains(CI->getValue()))
        return LazyValueInfo::True;

      if (CR.isSingleElement() && CR.contains(CI->getValue()))
        return LazyValueInfo::False;
    }

    // Handle more complex predicates.
    ConstantRange TrueValues =
        ICmpInst::makeConstantRange((ICmpInst::Predicate)Pred, CI->getValue());
    if (TrueValues.contains(CR))
      return LazyValueInfo::True;
    if (TrueValues.inverse().contains(CR))
      return LazyValueInfo::False;
    return LazyValueInfo::Unknown;
  }

  if (Result.isNotConstant()) {
    // If this is an equality comparison, we can try to fold it knowing that
    // "V != C1".
    if (Pred == ICmpInst::ICMP_EQ) {
      // !C1 == C -> false iff C1 == C.
      Res = ConstantFoldCompareInstOperands(ICmpInst::ICMP_NE,
                                            Result.getNotConstant(), C, DL,
                                            TLI);
      if (Res->isNullValue())
        return LazyValueInfo::False;
    } else if (Pred == ICmpInst::ICMP_NE) {
      // !C1 != C -> true iff C1 == C.
      Res = ConstantFoldCompareInstOperands(ICmpInst::ICMP_NE,
                                            Result.getNotConstant(), C, DL,
                                            TLI);
      if (Res->isNullValue())
        return LazyValueInfo::True;
    }
    return LazyValueInfo::Unknown;
  }

  return LazyValueInfo::Unknown;
}

/// getEdgeValue - This method attempts to infer more complex 
bool LazyValueInfoCache::getEdgeValue(Value *Val, BasicBlock *BBFrom,
                                      BasicBlock *BBTo, LVILatticeVal &Result) {
  // If already a constant, there is nothing to compute.
  if (Constant *VC = dyn_cast<Constant>(Val)) {
    Result = LVILatticeVal::get(VC);
    return true;
  }
  
  // TODO: Handle more complex conditionals.  If (v == 0 || v2 < 1) is false, we
  // know that v != 0.
  if (BranchInst *BI = dyn_cast<BranchInst>(BBFrom->getTerminator())) {
    // If this is a conditional branch and only one successor goes to BBTo, then
    // we maybe able to infer something from the condition. 
    if (BI->isConditional() &&
        BI->getSuccessor(0) != BI->getSuccessor(1)) {
      bool isTrueDest = BI->getSuccessor(0) == BBTo;
      assert(BI->getSuccessor(!isTrueDest) == BBTo &&
             "BBTo isn't a successor of BBFrom");
      
      // If V is the condition of the branch itself, then we know exactly what
      // it is.
      if (BI->getCondition() == Val) {
        Result = LVILatticeVal::get(ConstantInt::get(
                              Type::getInt1Ty(Val->getContext()), isTrueDest));
        return true;
      }
      
      // If the condition of the branch is an equality comparison, we may be
      // able to infer the value.
      ICmpInst *ICI = dyn_cast<ICmpInst>(BI->getCondition());
      if (ICI && ICI->getOperand(0) == Val &&
          isa<Constant>(ICI->getOperand(1))) {
        if (ICI->isEquality()) {
          // We know that V has the RHS constant if this is a true SETEQ or
          // false SETNE. 
          if (isTrueDest == (ICI->getPredicate() == ICmpInst::ICMP_EQ))
            Result = LVILatticeVal::get(cast<Constant>(ICI->getOperand(1)));
          else
            Result = LVILatticeVal::getNot(cast<Constant>(ICI->getOperand(1)));
          return true;
        }

        if (ConstantInt *CI = dyn_cast<ConstantInt>(ICI->getOperand(1))) {
          // Calculate the range of values that would satisfy the comparison.
          ConstantRange CmpRange(CI->getValue(), CI->getValue()+1);
          ConstantRange TrueValues =
            ConstantRange::makeICmpRegion(ICI->getPredicate(), CmpRange);

          // If we're interested in the false dest, invert the condition.
          if (!isTrueDest) TrueValues = TrueValues.inverse();
          
          // Figure out the possible values of the query BEFORE this branch.  
          if (!hasBlockValue(Val, BBFrom)) {
            BlockValueStack.push(std::make_pair(BBFrom, Val));
            return false;
          }
          
          LVILatticeVal InBlock = getBlockValue(Val, BBFrom);
          if (!InBlock.isConstantRange()) {
            Result = LVILatticeVal::getRange(TrueValues);
            return true;
          }

          // Find all potential values that satisfy both the input and output
          // conditions.
          ConstantRange PossibleValues =
            TrueValues.intersectWith(InBlock.getConstantRange());

          Result = LVILatticeVal::getRange(PossibleValues);
          return true;
        }
      }
    }
  }

  // If the edge was formed by a switch on the value, then we may know exactly
  // what it is.
  if (SwitchInst *SI = dyn_cast<SwitchInst>(BBFrom->getTerminator())) {
    if (SI->getCondition() == Val) {
      // We don't know anything in the default case.
      if (SI->getDefaultDest() == BBTo) {
        Result.markOverdefined();
        return true;
      }
      
      // We only know something if there is exactly one value that goes from
      // BBFrom to BBTo.
      unsigned NumEdges = 0;
      ConstantInt *EdgeVal = 0;
      for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i) {
        if (SI->getSuccessor(i) != BBTo) continue;
        if (NumEdges++) break;
        EdgeVal = SI->getCaseValue(i);
      }
      assert(EdgeVal && "Missing successor?");
      if (NumEdges == 1) {
        Result = LVILatticeVal::get(EdgeVal);
        return true;
      }
//.........这里部分代码省略.........

LVILatticeVal LVIQuery::getBlockValue(BasicBlock *BB) {
  // See if we already have a value for this block.
  LVILatticeVal BBLV = getCachedEntryForBlock(BB);
  
  // If we've already computed this block's value, return it.
  if (!BBLV.isUndefined()) {
    DEBUG(dbgs() << "  reuse BB '" << BB->getName() << "' val=" << BBLV <<'\n');
    return BBLV;
  }

  // Otherwise, this is the first time we're seeing this block.  Reset the
  // lattice value to overdefined, so that cycles will terminate and be
  // conservatively correct.
  BBLV.markOverdefined();
  Cache[BB] = BBLV;
  
  Instruction *BBI = dyn_cast<Instruction>(Val);
  if (BBI == 0 || BBI->getParent() != BB) {
    LVILatticeVal Result;  // Start Undefined.
    
    // If this is a pointer, and there's a load from that pointer in this BB,
    // then we know that the pointer can't be NULL.
    bool NotNull = false;
    if (Val->getType()->isPointerTy()) {
      for (BasicBlock::iterator BI = BB->begin(), BE = BB->end();BI != BE;++BI){
        LoadInst *L = dyn_cast<LoadInst>(BI);
        if (L && L->getPointerAddressSpace() == 0 &&
            L->getPointerOperand()->getUnderlyingObject() ==
              Val->getUnderlyingObject()) {
          NotNull = true;
          break;
        }
      }
    }
    
    unsigned NumPreds = 0;    
    // Loop over all of our predecessors, merging what we know from them into
    // result.
    for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
      Result.mergeIn(getEdgeValue(*PI, BB));
      
      // If we hit overdefined, exit early.  The BlockVals entry is already set
      // to overdefined.
      if (Result.isOverdefined()) {
        DEBUG(dbgs() << " compute BB '" << BB->getName()
                     << "' - overdefined because of pred.\n");
        // If we previously determined that this is a pointer that can't be null
        // then return that rather than giving up entirely.
        if (NotNull) {
          const PointerType *PTy = cast<PointerType>(Val->getType());
          Result = LVILatticeVal::getNot(ConstantPointerNull::get(PTy));
        }
        
        return Result;
      }
      ++NumPreds;
    }
    
    
    // If this is the entry block, we must be asking about an argument.  The
    // value is overdefined.
    if (NumPreds == 0 && BB == &BB->getParent()->front()) {
      assert(isa<Argument>(Val) && "Unknown live-in to the entry block");
      Result.markOverdefined();
      return Result;
    }
    
    // Return the merged value, which is more precise than 'overdefined'.
    assert(!Result.isOverdefined());
    return Cache[BB] = Result;
  }
  
  // If this value is defined by an instruction in this block, we have to
  // process it here somehow or return overdefined.
  if (PHINode *PN = dyn_cast<PHINode>(BBI)) {
    LVILatticeVal Result;  // Start Undefined.
    
    // Loop over all of our predecessors, merging what we know from them into
    // result.
    for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
      Value* PhiVal = PN->getIncomingValueForBlock(*PI);
      Result.mergeIn(Parent.getValueOnEdge(PhiVal, *PI, BB));
      
      // If we hit overdefined, exit early.  The BlockVals entry is already set
      // to overdefined.
      if (Result.isOverdefined()) {
        DEBUG(dbgs() << " compute BB '" << BB->getName()
                     << "' - overdefined because of pred.\n");
        return Result;
      }
    }
    
    // Return the merged value, which is more precise than 'overdefined'.
    assert(!Result.isOverdefined());
    return Cache[BB] = Result;
  }

  assert(Cache[BB].isOverdefined() && "Recursive query changed our cache?");

  // We can only analyze the definitions of certain classes of instructions
//.........这里部分代码省略.........

bool LazyValueInfoCache::solveBlockValue(Value *Val, BasicBlock *BB) {
  if (isa<Constant>(Val))
    return true;

  if (hasCachedValueInfo(Val, BB)) {
    // If we have a cached value, use that.
    DEBUG(dbgs() << "  reuse BB '" << BB->getName()
                 << "' val=" << getCachedValueInfo(Val, BB) << '\n');

    // Since we're reusing a cached value, we don't need to update the
    // OverDefinedCache. The cache will have been properly updated whenever the
    // cached value was inserted.
    return true;
  }

  // Hold off inserting this value into the Cache in case we have to return
  // false and come back later.
  LVILatticeVal Res;

  Instruction *BBI = dyn_cast<Instruction>(Val);
  if (!BBI || BBI->getParent() != BB) {
    if (!solveBlockValueNonLocal(Res, Val, BB))
      return false;
   insertResult(Val, BB, Res);
   return true;
  }

  if (PHINode *PN = dyn_cast<PHINode>(BBI)) {
    if (!solveBlockValuePHINode(Res, PN, BB))
      return false;
    insertResult(Val, BB, Res);
    return true;
  }

  // If this value is a nonnull pointer, record it's range and bailout.
  PointerType *PT = dyn_cast<PointerType>(BBI->getType());
  if (PT && isKnownNonNull(BBI)) {
    Res = LVILatticeVal::getNot(ConstantPointerNull::get(PT));
    insertResult(Val, BB, Res);
    return true;
  }

  // If this is an instruction which supports range metadata, return the
  // implied range.  TODO: This should be an intersection, not a union.
  Res.mergeIn(getFromRangeMetadata(BBI), DL);

  // We can only analyze the definitions of certain classes of instructions
  // (integral binops and casts at the moment), so bail if this isn't one.
  LVILatticeVal Result;
  if ((!isa<BinaryOperator>(BBI) && !isa<CastInst>(BBI)) ||
     !BBI->getType()->isIntegerTy()) {
    DEBUG(dbgs() << " compute BB '" << BB->getName()
                 << "' - overdefined because inst def found.\n");
    Res.markOverdefined();
    insertResult(Val, BB, Res);
    return true;
  }

  // FIXME: We're currently limited to binops with a constant RHS.  This should
  // be improved.
  BinaryOperator *BO = dyn_cast<BinaryOperator>(BBI);
  if (BO && !isa<ConstantInt>(BO->getOperand(1))) { 
    DEBUG(dbgs() << " compute BB '" << BB->getName()
                 << "' - overdefined because inst def found.\n");

    Res.markOverdefined();
    insertResult(Val, BB, Res);
    return true;
  }

  if (!solveBlockValueConstantRange(Res, BBI, BB))
    return false;
  insertResult(Val, BB, Res);
  return true;
}